The present invention concerns a method for making a display module including at least one liquid crystal cell connected to a control printed circuit for the display whose electronic components for controlling the cell and whose metal connection paths to said components and said cell are arranged on a single face. The invention also concerns a display module which can be obtained according to the method, as well as a watch including a display module.
Liquid crystal displays are used in various fields, for example for calculators or for devices displaying various data, which are linked to one or more printed circuits with electronic components for controlling the electrodes of the cell. Sometimes, several electronic components for controlling the cell are placed on a plate of the cell which also carries the conductive pads connected to the electrodes. Usually, the surface defined by the cell connected to the display control printed circuit or circuits located next to the cell is often too large to be able to be placed in a case of an apparatus of small dimensions, in which it is necessary to have compact display modules.
The printed circuits for said cells are usually rigid and more often their metal output pads are electrically connected to the respective conductive pads of the liquid crystal cell by zebra connectors inserted between said pads. The contact pressure necessary for this type of connection induces a certain mechanical tension between the cell, its support and the printed circuit, which may be a drawback. No great importance is generally attached to the location of the printed circuit, whether it is below or next to the liquid crystal cell to be connected. It is only in modular embodiments that the printed circuit is usually placed below the cell with, in this case, said zebra connectors forming part of the spacer elements.
U.S. Pat. No. 5,358,412 discloses a modular embodiment as mentioned above. The display module includes a module frame for enclosing a liquid crystal cell, a spacer element and a rigid printed circuit including all the electronic control components for said cell. The printed circuit can be a single-face printed circuit on which a zebra connector forming part of the spacer element is compressed in the frame to connect on one side the metal output pads of the printed circuit and on the other side conductive pads connected to the electrodes of the liquid crystal cell. The display module is mechanically and electrically connected to a flexible printed circuit in particular to supply said module with electricity. The electric connection between this flexible printed circuit and the module occurs on the surface of the rigid printed circuit carrying metal output paths of the module. No glue is used to maintain the electric connection, but the end of the flexible circuit is pressed between the spacer element and the metal output paths arranged on the rigid printed circuit.
In one embodiment, said flexible printed circuit includes two openings which fit onto two pillars of the rigid printed circuit to allow, in particular, precise positioning for the electric connection. It is to be noted that the frame is used for mounting all the elements of said module, as well as for connecting the flexible printed circuit to said module.
Given that, in one embodiment of said module, all the metal paths and all the cell control components are arranged on a single face of the rigid printed circuit in the direction of the back of the cell, a gap inevitably remains between the electronic components and the back of the liquid crystal cell. Likewise, in the event that the components are placed on an opposite face of the rigid printed circuit, gap remains because a connecting bar is used between metal pads arranged in this case on the back of the rigid printed circuit and respective metal pads of the liquid crystal cell.
The use of flexible circuits including metal paths, as well as liquid crystal cell control electronic components is also known. U.S. Pat. No. 5,469,412 discloses an analogue watch for detecting nuclear radiations. A flexible double face printed circuit is described. Two tongues carrying metal pads are directly connected to respective conductive pads of two liquid crystal display cells without using zebra connectors. This circuit is positioned under a frame including the watch movement, while the tongues are bent so that the liquid crystal cells are placed on this frame. Another part of the flexible printed circuit is connected to the watch detector and bent in order to be placed under an intermediate plate in an opposite direction to the cells.
Japanese Pat. No. 56031684, which concerns a design for a liquid crystal cell watch may also be cited. In this document, it may be noted that the single-face flexible printed circuit is first electrically connected to said cell without zebra connectors, then bent in the shape of an accordion under said cell. This type of construction creates a certain difficulty for mounting the assembly in a watch case so as to guarantee proper positioning of said display. In order to do this, one has to use a frame in which the liquid crystal cell is housed in order to have a display which is well centred with respect to the watch case.
The object of the present invention is to obviate the drawbacks of the aforementioned display modules. It proposes manufacturing a compact display module of small thickness, arranged so that the module can be easily and precisely mounted in a case of an apparatus in order for the display to be in a determined position. Said module can be mounted in particular in a wristwatch case.
One embodiment solution for said module consists in placing the printed circuit on the back side of the cell, in positioning it so that module assembly means made on the printed circuit define, with respect to display marking means arranged on the cell, the location of the liquid crystal cell, and in securing the face of the printed circuit which has no components on the back of the cell according to the characteristic part of claim 1.
Another embodiment solution for said module consists in placing the printed circuit on the back side of the cell, in fixing the face of the printed circuit which has no components on the back of the liquid crystal cell, and in forming means for assembling the module on the printed circuit, positioned with respect to display marking means arranged on the cell, so that they define the location of the cell display according to the characteristic part of claim 2.
One of the advantages of the module obtained via this method, which is the subject of the invention, is that the thickness of the cell with the single-face printed circuit, whose face which has no components is directly secured to the back of said cell to make it compact, is reduced to a minimum so as to be able to be mounted, for example in a digital watch of small thickness. It is of course clear that this thickness is largely dependent upon that of said cell which is a component obtained on the market and whose thickness is close to 1 mm. One may envisage in the near future obtaining such cells with a thickness of less than 1 mm.
Another advantage of the module according to the invention is that, because the printed circuit has all the electronic components and metal paths on a single surface of said printed circuit, the manufacturing costs are reduced to a minimum by avoiding the creation of metallised holes through the printed circuit to establish an electric connection of the metal paths and the components which would be arranged on both faces. Mounting of the electronic components on the printed circuit is also facilitated. Moreover, the arrangement of the metal paths is achieved so that said paths do not intersect over the entire printed circuit. The output terminals of the integrated circuit for controlling the cell therefore accord with the respective cell terminals to be contacted so that the metal paths from the output terminals of the integrated circuit do not intersect up to the connection to the cell.
Given that the electric connection of the printed circuit with the cell is achieved without intermediate elements, in particular without zebra connectors, the spacing between the pads or metal connection terminals of the printed circuit, and between the conductive paths of the cell may be reduced. The spacing, i.e. the pitch which is the added width of a path and a gap, could thus be less than 0.5 mm, and even less than 0.3 mm, but this depends on what is currently provided on the market by way of liquid crystal cells. With this reduced spacing, one can position precisely the tongue of the circuit carrying the metal pads for connection to the cell above the respective conductive pads of the cell and connect said pads electrically using an anisotropic conductive adhesive material which only becomes conductive when it is compressed and heat is applied thereto.